Textures black out after remesh?
Quadriflow remesh will remove any UVs you have, one solution is to use Data Transfer modifier on the remeshed version, to transfer UVs from the photoscan to the remeshed version, use settings in the image below, you should also place both meshes in the same location, and preferably hide the photoscan
Another solution is to use the Remesh Modifier instead of quadriflow at the cost of low topology quality.